If you are not using LLVM, your main project will not be able to use ARC as a function of LLVM 3.0.
If I were you, I would compile your main project / target / application under LLVM and include your old external dependencies as dependencies of the static library. Once the static libraries are compiled, the fact that they are ARC or not ARC does not matter.
Xcode, Xcode, . . .
Xcode, .